Part L – or Approved Document L – came into effect on April 1 as part of the government’s multi-pronged attack on energy loss and carbon dioxide emissions. In setting out new fuel and power conservation requirements, the document has ramifications for manufacturers of products such as timber frame and timber windows and doors.

The rate at which heat is lost is measured in watts per square metre per degree Kelvin – simplified as the U value. For new build, windows and external doors must achieve an average whole window U value of 2.0 or less. Replacement windows can be treated as for new build or must use glass with a centre pane U value of 1.2 or less. Replace-ment doors in houses will need to meet the requirement only where they have 50% or more glass area, which must then be proved by hot-box testing, calculation or by using glass with a centre pane U value of 1.2.

It may be possible to ‘trade off’ individual door and window values to bring the average for all doors and windows in a building below the 2.0 U value threshold.

There are three methods for producing U values: by reference to the table of default values given in Part L; by using ISO standard methods of calculation; and by using U values obtained from hot-box testing in a UKAS-approved laboratory.

Chiltern Dynamics offers U value simulation enabling product designs to be ‘tweaked’ to achieve desired performance for the lowest cost.

Higher quality windows

The British Woodworking Federation (BWF), producer of a fact card explaining Part L, has welcomed the new measures. ‘It means higher quality windows and doors will have to be sold more,’ said Adam Frankling, product manager for the BWF’s Timber Window Accreditation and Certifire schemes. ‘The next big challenge for manufacturers is to inform customers and consumers about the existence of the regulations and the cost implications.’

Specially coated glass – or ‘e’ glass – used to reflect heat from a building back inside would add as much as 60% to the glass element of the overall product price. Despite all the pre-publicity for Part L, the need for higher prices will still come as a shock to some builders.

Another technical expert from the manufacturing sector believed Part L could ‘revolutionise’ the standard timber door market because of limitations on the maximum thickness of double glazing, adding that the regulations tended to favour composite and foam filled products in new build. Part L would also make factory glazing and finishing ‘the norm rather than just popular’, he suggested, because the compliance burden fell on the building owner who would generally prefer to be able to trace products straight back to the manufacturer.

According to Hugh McKay, chairman of Stewart Milne Timber Systems and board member of the UK Timber Frame Association (UKTFA), the advent of Part L was good news for the timber frame sector since, according to his company’s calculations for Scotland, it will add only £200 to the cost of building a timber frame house against £600 for a brick house.

In terms of the impact of Part L on timber frame, there would be a gradual move towards increasing stud dimensions since ‘this is a fairly easy operation for the industry and the most cost-effective way of meeting the government’s requirements,’ predicted Mr McKay. He anticipated a general move from 90mm to 140mm over the next two years. While the volume sector may take some time to effect the change, ‘the one-off builder should be thinking of moving to 140mm now’, he added.

Part L would also favour a move towards pre-finished factory panel systems and would accelerate developments of a more closed panel-type system ‘to be able to incorporate new types of insulation coming onto the market,’ noted Mr McKay.

Guidance document

UKTFA has instructed the BRE to produce a guidance document clarifying the basic formulae for calculating U values, constants to be used and methods of achieving these U values.

Due to be available in electronic form by the end of April, the document will provide manufacturers with information to send out to architects, engineers and other professionals, as well as customers such as developers and housing associations.